The present invention relates to hydraulic flow control valves which limit flow at certain levels regardless of the pressure drop thereacross. With the use of hydraulic motors to drive sweeper brooms, combine reels, or any application requiring a set speed, it becomes more apparent that the present state of the art of adjustable flow limiting valves is inadequate. A conventional flow control valve, such as shown in U.S. Pat. No. 3,979,908 (also known in the trade as flow limiting valves), senses a pressure drop across a sensing orifice in the flow path and with that pressure drop balanced against a spring, controls a variable orifice in the flow path to maintain a constant pressure drop across that sensing orifice regardless of the changes of load on the system.
The flow limiters of the prior art can be designed to properly function at a single sensing orifice size. However, they do not efficiently function when the size of the sensing orifice is changed, because the spool closes due to certain dynamic forces on the spool aside from the spring force and servo chamber forces thereon. These dynamic forces include an increased momentum exchange on the valve spool, sometimes also referred to as "Bernoulli effect". These dynamic forces are described in detail in U.S. Pat. No. 4,220,178. The dynamic force must be balanced against the spring force not only in magnitude but in rate of change, and this can be accomplished only at one orifice size in a particular valve.